Recently, as mobile communication industries and information electronic industries progress in various technologies, a light-weight, high-capacity lithium secondary battery is increasingly in demand. However, a lithium secondary battery may ignite and explode due to extreme heat emission when it is over-charged or is in a short circuit state. Moreover, when a lithium secondary battery is overdischarged below a normal voltage range, its capacity is significantly reduced, preventing forthcoming use.
For these reasons, a safety device like a protection circuit, a PTC element, etc., has been attached to a lithium secondary battery since lithium secondary batteries were first developed. However, such protection circuits, PTCs, etc., are not preferable because they are expensive and take up a large volume, thereby increasing the price, volume and weight of a battery. Therefore, batteries with a reduced manufacturing cost and an increased battery capacity without using such a protection circuit, PTC, etc., are very much in demand.
Conventionally, an organic or an inorganic additive is used in a non-aqueous electrolyte, or the outer structure of a battery is changed for the purpose of ensuring battery safety when a battery is over-charged or has short-circuited. However, when a battery is overdischarged below an adequate voltage, even if one tries to charge the battery again, the battery capacity is so significantly reduced that the battery are no longer capable of charge/discharge.
Conventional lithium secondary batteries developed hitherto have a structure in which discharge is limited and terminated by an anode during overdischarge. Particularly, when a non-aqueous lithium secondary battery is first charged, a solid electrolyte interface (SEI) film is formed on the surface of an anode. In this case, a great amount of lithium ions released from a cathode are used and thus the amount of Li participating in charge/discharge is reduced. When over-discharging occurs in the state in which the amount of Li is reduced, activated Li sites in the cathode are not fully occupied and the cathode voltage is not decreased below a certain voltage. Therefore, discharge is terminated by the anode (see FIG. 1).
Meanwhile, a battery capacity is significantly reduced by the following reasons. A battery voltage is defined by a difference between a cathode voltage and an anode voltage. Additionally, a battery is continuously discharged at a low electric current, even after the battery voltage is decreased below a general-use voltage. At this case, due to the consumption of Li ion in the anode, the cathode voltage is no longer reduced and thus it is slowly decreased. On the other hand, the anode voltage rapidly increases and eventually rises to 3.6 V, at which point a copper foil used as an anode collector is oxidized. Thus, the copper foil is dissolved in a copper ion state to contaminate an electrolyte. After that, when the battery is re-charged the copper ion is attached again to the surface of the anode and thus the anode active material becomes unusable. Therefore, if oxidization of the copper foil occurs, the battery capacity is rapidly reduced after overdischarge, so that the battery becomes unusable.
Accordingly, it is desirable to develop a battery, discharge of which is limited by a cathode, so that the battery capacity may not be significantly reduced after overdischarge. Further, a new method for making such a cathode-limited battery is very much in demand.